The bridge wall should come up to within 12 or 14 inches of the bottom of the boiler, and be curved to suit its form, although this is not absolutely necessary. The width of grate surface should be equal to the diameter of the boiler.

The side walls of the furnace are to incline outwards, so as to be two inches from the sides of the boiler, at a point one course of bricks below the bottom of the brackets. The fire bricks should be laid with a header course every five courses, so that burned-out bricks may be conveniently replaced.

At each side of the fire doors cast iron "cheek-pieces" should be put in, the cast iron arch plate over the door resting on them. These "cheek-pieces" should be about 1 inches thick, of the form shown in Fig. 77, and have as many half-inch holes cored in them as possible, the holes spaced two inches from center to center, for an air supply to prevent them from burning out. Their height is equal to the height of the fire door at the side. They will be found to be very durable and to save much expense in fire brick repairs.

FIG. 76. Vertical Half Cross Section and
Elevation of Boiler Setting.

They may be removed and replaced whenever the furnace is cool, by jacking up the arch plate a trifle and letting it down on the new "cheek-piece"

introduced. Their inclined form renders the cleaning of the fire much more convenient and the extreme front corner which they cut off is of little benefit in making steam.

Both outside and division walls should have a two-inch air space, as shown. The top of the boiler should be covered with asbestos, or with a brick arch. If the latter, there should be a two-inch air space left between it and the boiler. The boilers must rest only on the supporting brackets and in no case on the boiler fronts.

The fronts are held in place by anchor bolts 3 inch in diameter and 4 feet long, with the inner ends bent to a right angle 10 or 12 inches long. Their front ends are threaded for nuts coming outside of the boiler fronts, so that a defective or cracked portion of the front may be readily removed and replaced without disturbing the brickwork.

The smoke connections from the boilers to the stack may be the same width all the way through, in which case its height is to be increased from the first to the sixth boiler to include the additional area necessary for each boiler as it progresses toward the stack. Thus it may be 36 inches wide and 20 inches high at the first boiler, and increasing to 78 inches high at the sixth. boiler. It will perhaps be more convenient to increase also the width, in order that the area at the large end may equal that of the stack without increasing the height to such an extent.

By this method we may make the larger end 48 inches wide and 60 inches high. There should be a cleaning door in the end of the smoke connection. at the first boiler, and a pivoted damper properly balanced between the sixth boiler and the stack. It will be convenient also to have dampers in the "uptake" from each boiler to the main smoke connection, so as to shut these off whenever a boiler is laid off for cleaning or repairs.

The steam connections from the tops of the boilers are to be so arranged

that any one or more of the boilers may be "cut out" and the use of all the others continued. All steam pipes of three inches or over should be covered with an efficient and lasting non-conducting material. That containing a large portion of asbestos will probably be found the best.

The foundations for the boiler settings should be prepared in the same manner as for a machine foundation, as described in Chapter X, due consideration being given to the weight to be supported, say about 1,200 pounds per square foot.

The plan of boiler settings shown, that is, supporting the boiler on brackets attached to it, is the ordinary method. The more modern method, however, is to erect iron columns at each side of the boiler and upon these to lay I-beams, from which the boiler is suspended by iron rods, entirely clear of the brickwork, and with no part resting upon it. While this plan is no doubt correct in theory and practice, it is considerably more expensive than the method shown herewith and for that reason it may not receive the favor it deserves.

The general plan and arrangement of the boiler room with the boiler settings, the smoke connections with the stack, the coal-delivering tram track, scales, etc., and the engine room, with the location of the engine and its connection with the main shaft, is shown in Fig. 78, and in so far as it relates to the boilers and settings it is substantially the system adopted by the Bigelow Company, New Haven, Conn.

As to the engine, it seems fairly well conceded that for economy of steam and general efficiency in furnishing the power for machine shop work the horizontal, cross compound condensing engine will be the best. This type of engine is made by a number of well-known engine builders, and while all of them have certain convenient features and peculiarities of design and construction which commend them to different purchasers, it is probable that there is no very great difference in their efficiency or economy in the general results.

The subject of gas engines has not been considered in connection with our plans as they do not seem suitable where a large amount of power is required, whatever may be their advantages in small or isolated plants, although gas engines have been built as large as a thousand horse-power that have been fairly successful.

Prominent among the builders of steam engines of the type referred to above are the Allis-Chalmers Company, of Milwaukee, Wis., and the William A. Harris Engine Company, of Providence, R. I., and it is this type of engine which is illustrated and described in this article, and commended for machine shop use.

The size selected may be 16 and 32 x 48 inches, or 18 and 36 x 42 inches, with a balance wheel 18 feet in diameter with a 36-inch face, and capable of

generating 400 horse-power at 80 revolutions per minute, and steam at 125 pounds pressure.

It will be proper to consider whether to drive direct from the engine to the machines by means of shafting and belts, or whether the engine shall drive dynamos, from which the electric current may be transmitted by proper

conductors to motors, which in turn may drive main lines of shafting, or to a number of motors located in different parts of the works, driving short lines of shafting operating groups of machines, or again, whether we shall place a small motor upon each individual machine to drive it.

All of these methods have their peculiar advantages and necessarily their

disadvantages, corresponding to the conditions, the positions of the machines. to be operated and the duty that is to be performed. Again, we may profitably make use of compressed air for some of our work, as for instance, for drawing patterns, turning flasks and other portions of the lighter work of the iron foundry, as well as for the chipping room where hand chipping tools so operated are very convenient and useful.

Hammers operated by compressed air may be used in the forge shop, since this force may be transmitted long distances with practically no loss such as steam is subjected to by condensation, and electricity by loss of electromotive force.

Various operations in the machine shop also may render a supply of compressed air very desirable. This matter will be governed to a considerable extent by the kind of work to be done, or kind of machinery to be built.

It would seem best in theory as well as practice, and most efficient and economical, to avail ourselves of whatever good points each method possesses for the particular case in question, and, by using any of the different systems where the conditions are most favorable for its employment, to make the most of the useful and practical features and avoid as many of the difficulties and disadvantages as we may be able.

For instance, while the practice of driving individual machines by separate motors may be said to be yet in its infancy, enough has been already done to prove its advisability in many ways, and to show that planers from 40 inches upwards may be profitably driven in this manner.

The same may be said of lathes from say 36 inches upwards, and also of the larger radial drills, vertical milling machines, boring mills, and, in fact, of most of the heavy machine shop tools. At the same time it does not appear to be as efficient or practical to apply individual motors to small machines when a group of them may be conveniently driven from a short line shaft run by one motor. The more recent improvements in motors, however, have adapted them to their economical use on much smaller machines even when a slow speed is required.

The question of friction of the two systems of transmitting power, that is, from the engine by shafting and belting, or the loss of power by generating an electric current with which to drive motors, is one which has provoked much discussion. Probably it will be found in practice to be about as follows: Where the distance is short, shafting and pulleys are much the more economical. For distances of two or three hundred feet there will be little difference in the two systems. For much greater distances the advantages are in favor of the electric method.

The plan of power transmission here selected is to drive from the balance wheel of the engine to a 72-inch pulley on the main line of shafting, giving a